Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181711
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1111 AM MDT SUN SEP 18 2016

.SHORT TERM...Sunday through Tuesday night

Fire weather will be the main concern today as we are in a squeeze
play across the area with a cold front and shortwave to the north
and high pressure to the south. The result is a tight pressure
gradient that will bring breezy to windy conditions to much of the
area. Chief Joseph has already had some gusts over 60 mph Saturday
night. Guidance has sustained winds over 25 knots and with 35 to 40
knots of wind aloft mixing to the surface, this seams reasonable.
The limiting factor will be relative humidity. Most areas will fall
short of the 15 percent for three hours, although a few areas may
reach it. For now, we will go with headlines for near critical fire
conditions. As for the chance of precipitation, there will be some
showers across the north, mainly in the mountains. However, with
little moisture to work with we kept POPS to the isolated to widely
scattered variety.

Monday at this point looks to be a drier day with heights building
across the area as flat ridging moves over the area. As a result,
relative humidity will be a bit lower and temperatures a bit warmer.
However, with somewhat lighter winds aloft and the pressure gradient
a bit less, it should not be as windy as Sunday. There could be near
critical fire conditions in some areas however. And showers would be
limited to the northern mountains and would likely be few and far
between.

With flow turning more to the southwest for Tuesday, this looks to
be the warmest day of the short term period. The models are
indicating some showers associated with another shortwave heading
toward the area. The keep these mainly over Montana however so we
have elected to keep the forecast dry.

.LONG TERM...Wednesday through Sunday

An area of low pressure moves over the pacific northwest by Wednesday
morning. This system will keep southwest flow over the area, with
increased moisture moving over the state. The western mountains and
valleys look most favorable for shower activity Wednesday, with
activity increasing through the evening and overnight hours. Low
pressure then moves eastward, bringing the potential for more
widespread shower activity as the system approaches western
Wyoming. Models supportive of this increase with decent agreement
noted between models.

Models vary a bit with the speed of the low as it attempts to move
across Wyoming on Thursday and Friday. The fastest solution is the
Canadian, which has it centered around Sublette/Teton counties by
late Thursday evening. GFS and ECMWF are slower with the
progression, keeping the low west of the state border through Friday
morning. The GFS continues to push the low further south across
northern Utah by Friday morning, while the ECMWF moves along a
similar path as the Canadian, moving the low through the state.
Shower coverage will vary based on which solution is correct, though
regardless showers and cooler temperatures should be fairly
widespread. Mountain snows are expected, with snow levels
approaching the floors of the western valleys by late Thursday
night.

The low pressure is expected to lift through Saturday, with the
center located over southwestern Montana by Saturday afternoon. As
the system continues to lift north, building northwest flow is
expected Saturday evening through Sunday, with increased dry
conditions.

&&

.AVIATION.../18Z ISSUANCE/

VFR conditions will prevail through the forecast period. Some
isolated showers will occur at times over the northwest mountains
through tonight. VCSH at the KJAC terminal through 10z tonight and
at KCOD this evening. A few showers are also expected  over the
northern Bighorn mountains through tonight. Most locations in
Wyoming will be breezy to windy today with terminals seeing wind
gusts between 25 and 35 knots this afternoon and early evening. The
wind will decrease between 01z and 02z for most locations. Wind
shear will the be an issue for most terminal sites this at the very
least this evening...and at many locations through the early morning
hours.

&&

.FIRE WEATHER...

...NEAR CRITICAL FIRE CONDITIONS FOR MANY AREAS THIS AFTERNOON...

With the area caught between an upper level disturbance to the north
and high pressure to the south, breezy to windy conditions will be
found across much of the area with gusts to 40 mph or higher
possible. Although relative humidity values are not expected to fall
below 15 percent in most areas, elevated fire danger is likely for
many areas this afternoon and early this evening. Smoke dispersal
and mixing will be good to excellent. A few showers are possible
across the northern mountains.
&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...Hattings
LONG TERM...Branham
AVIATION...Braun
FIRE WEATHER...Hattings



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